Typical interferometer systems require a large footprint on the target to measure a single axis of displacement. To measure displacement and two rotational degrees of freedom, three identical interferometer systems are typically needed by the prior art. This greatly increases cost and makes the overall footprint of the interferometer very large. Standard displacement interferometer systems also typically manipulate the polarization states of the optical beams with both the reference and measurement arms in the interferometer with several overlapping instances prior to the final interfering surface. This leads to the measurement of nonlinearity as a function of target position, commonly called periodic nonlinearity. This results from using a heterodyne source with nominally orthogonally polarized, collinear optical beams that suffer from imperfect optics and imperfect alignment. Optical symmetry is typically maintained between measurement and reference arms by having a complex optical path through numerous optical components. This increases susceptibility to thermal gradients and requires more costly optical components to address these deficiencies.
The art lacks a balanced, periodic error free, three degrees of freedom measuring interferometer.